Electrochemical preparation of metal sulfides



United States Patent 3,375,180 ELECTROCHEMICAL PREPARATION OF METAL SULFIDES Harold J. Heinen, Don H. Baker, Jr., and John M. Gomes, Reno, Nev., assignors to the United States of America as represented by the Secretary of the Interior N0 Drawing. Filed Apr. 2, 1965, Ser. No. 445,280

- 11 Claims. (Cl. 20461) The invention herein described and claimed may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of royalties thereon or therefor.

This invention relates to preparation of high-purity, crystalline Group VI-B metal sulfides, particularly molybdenum and tungsten sulfides.

Molybdenum disulfide is used as an ingredient in solid film lubricants at temperatures up to 500 F. Preparation of M08 ore concentrates for this use is usually limited to the chemical removal of the abrasive silicious matter. Further purification requires several complicated and costly steps. Consequently, currently available MoS materials contain appreciable iron contamination which is susceptible'to oxidation and is deleterious in lubricants. The synthetic molybdenum sulfides obtained by interaction of molybdic oxide and sulfur in saline melts, or by other chemical methods, are amorphous or cryptocrystalline powders. These powders differ from the natural molybdenite in crystal structure and are inferior lubricants in the dry state. Prior to our invention no high purity massive crystalline MoS having identical crystal structure to that of natural molybdenite was ever synthesized.

Synthetic W5 is also used as a high temperature lubricant (up to 700 C.) and as a catalyst. The usual synthetic material differs from the natural W8 in crystal structure. There are no known commercial sources of tungstenite (W8 a mineral found in only a few localities.

It is, therefore, an object of the present invention to provide a reliable, economical process for synthesizing high-purity Group VI metal sulfides having crystal structures essentially the same as the corresponding naturally occurring sulfides.

It has now been found that this object may be achieved by means of electrolysis of a molten salt bath containing a suitable source of the Group VI-B metal and sulfur.

A particularly suitable source of these materials is a mix-,

ture of the corresponding metal oxide and free sulfur. In the case of molybdenum and tungsten, to which the invention has been found particularly applicable, a mixture of M00 or W0 and free sulfur is used.

Other suitable sources of the metal and sulfur are flotation concentrates of the naturally occurring mineral, e.g., in the case of molybdenum the source is molybdenite (M08 while the tungsten source is tungstenite (W8 The method of the invention thus also affords a means for obtaining high-purity products from the impure naturally occurring minerals.

Processes for electrodeposition from fused salt baths are conventional and a wide variety of materials have been used to form the molten bath in which the electrolysis is conducted. The molten salt electrolyte components used in the instant invention are conventional; however, particular compositions may be optimum for deposition of a given metal sulfide.

For synthesis of M03 a particularly effective molten salt bath composition consists of a mixture of an alkali metal borate, an alkali metal halide and an alkali metal carbonate, The preferred composition of the solvent is a mixture of sodium carbon-ate, sodium tetraborate and "ice the eutectic mixture of sodium fluoride and potassium fluoride. The sodium carbonate is the principal reagent that dissolves the Mos -S mixture or the indigenous molybdenite. The presence of borate in the bath is conducive to deposition of massive foliated crystals of M08 the main function of the alkali halides being to provide the conductive medium for electrolysis.

Although Na CO has been found to be a very efiective solvent for impure M08 or M00 and S in the molten bath, it may be replaced in part or whole by other alkali metal salts such as hydroxides, peroxides, carbonates and bicarbonates. The borate component may be alkali metal tetraborate, pentaborate, metaborate or boric anhydride.

In this particular molten bath, the molar ratio of Na CO to M00 or M08 is preferably about 4.2 to 1.0. If insufficient N-a CO is employed, the M00 and S will interact to form insoluble amorphous molybdenum sulfides in the melt; if an excess of Na CO' is used, dimolybdenum carbide (Mo C) is deposited. Concentrations of M00 or MoS (impure) may vary over a wide range, e.g., about 2 to 30 weight percent; however, concentrations of about 5 to 10 percent are preferred. Concentration of borate and halide salts are also not critical and are best determined experimentally.

Temperatures of the molten bath may also vary considerably; however, about 900 to 1000 C. is generally preferred. At lower temperatures resulting electrowon MoS tends to be amorphous rather than crystalline.

The preferred initial cathode cur-rent density is from about 10 to 1 00 amperes per square decimeter. Anode current density is not critical and is usually maintained at about one-third to one-fifth of that of the cathode.

Cathode and anode materials are those conventionally used in fused salt electrolytic processes. The cathode may be any suitable conductive material such as graphite, tungsten, molybdenum, titanium, etc. The anode is preferably graphite or carbon.

For synthesis of W8 the preferred fused bath composition comprises potassium aluminum fluoride (KAlF and sodium fluoride (NaF). Concentrations are not critical, molar ratios of KAIF and NaF to W0 or WS of about 0.8 to 1.0 and 17 to 1.0 generally giving good results. Sodium aluminum fluoride may be substituted for all or part of the potassium aluminum fluoride.

The following examples will serve to more particularly illustrate the invention.

Example 1 The electrolyte consisted of 40.0 percent Na B O' 16.8 percent NaF, 12.4 percent KF, 20.9 percent Na CO 6.6 percent M00 and 3.3 percent S by weight. The mixed charge was heated externally in a graphite crucible that served also as the anode during electrolysis. The electrolysis was conducted at 1,000 C. and at an initial cathode current density of '60 amp/dmP. No protective atmosphere was used. The synthesized M08 was deposited on the cathode as a bulky honeycomb-like mass of foliated hexagonal crystals. The electrodeposit was withdrawn from the molten bath, air cooled, and then submerged in water to dissolve the drag-out electrolyte. Typical yield was grams M08 per 100 ampere hours of electrolysis. Screen analysis indicated that about percent of the electrosynthesized M05 was plus IOO-mesh in size. Crystals up to /2 inch in size have been produced.

X-ray diffraction analysis showed that the crystal structure of the electrosynthesized M08 was the same as that of natural molybdenite. Analysis of the synthesized M08 is given in Table 1. As is apparent from the table, the principal impurity is carbon; however, this is not deleterious for use as a solid film lubricant. In fact, M08 graphite solid film lubricants are currently in wide use.

TABLE 1.ANALYSIS OF ELECTROSYNTHESIZED SUL' FIDES, IN PERCENT Element MOSz W32 I Not detected.

Example 2 In this example the procedure was the same as that of Example 1, except that 7.5% M08 molybdenite flotation concentrate) was used as the feed material in place of M and S. Comparative analysis of this reconstituted (synthesized )MoS and the molybdenite flotation concentrate used as feed is shown in Table 2.

TABLE 2 Element MOS; feed, Reconstituted M08 Percent Percent 1 Not determined. 9 Not detected.

Example 3 In this example W5 was synthesized from a molten bath consisting of weight percent ME, percent NaF, 2 0 percent W0 and 10 percent S. Yield of W8 was about 135 grams per ampere hours. Analysis of the synthesized WS is given in Table 1.

What is claimed is:

1. A method for production of a high-purity. Group VI-B metal sulfide comprising electrolyzing a molten salt bath containing a feed material from the group consisting of (l) a mixture of the corresponding Group VI-B metal oxide and free sulfur and (2) an impure Group VI- B metal sulfide.

2. Method of claim 1 in which the high-purity metal sulfide is molybdenum sulfide.

3. Method of claim 2 in which the feed material comprises of a mixture of molybdenum oxide and free sulfur.

4. Method of claim 2 in which the feed material comprises molybdenite.

5. Method of claim 2 in which the feed material comprises a mixture of molybdenum oxide, free sulfur and molybdenite.

6. Method of claim 2 in which the molten salt bath comprises an alkali metal borate, an alkali metal halide and an alkali metal carbonate.

7. Method of claim 1 in which the high-purity metal sulfide is tungsten sulfide.

8. Method of claim 7 in which the feed material comprises a mixture of tungsten oxide and free sulfur.

9. Method of claim 7 in which the feed material comprises tungstenite.

10. Method of claim 7 in which the feed material comprises a mixture of tungsten oxide, free sulfur and tungstenite.

11. Method of claim 7 in which the molten salt bath comprises an alkali metal aluminum halide and an alkali metal halide.

References Cited UNITED STATES PATENTS 1,795,512 3/1931 Schmidt 20461 3,330,646 7/ 1967 Heinen et al. 20461XR HOWARD S. WILLIAMS, Primary Examiner.

D. R. VALENTINE, Assistant Examiner. 

1. A METHOD FOR PRODUCTION OF A HIGH-PURITY. GROUP VI-B METAL SULFIDE COMPRISING ELECTROLYZING A MOLTEN SALT BATH CONTAINING A FEED MATERIAL FROM THE GROUP CONSISTING OF (1) A MIXTURE OF THE CORRESPONDING GROUP VI-B METAL OXIDE AND FREE SULFUR AN (2) AN IMPURE GROUP VI-B METAL SULFIDE. 